<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <div id="app">
    <input type="checkbox" value="a" v-model="arr" >a <br/>
    <input type="checkbox" value="b" v-model="arr" >b <br/>
    <input type="checkbox" value="c" v-model="arr" >c <br/>
    <input type="checkbox" value="d" v-model="arr" >d <br/>
  </div>
</body>
<script src="lib/vue.js"></script>
<script>
  new Vue({
    el: '#app',
    data: {
      arr: ['c']
    }
  })
</script>
</html>